We inset a vacuum of 18 Å in between sets of cells, to avoid interaction between replica images as a result of periodic boundary conditions. In order to describe the strong correlation of electrons in Mott-Hubbard physics, we adopted a DFT+U approach, where U is the on-site Coulomb repulsion, using the simplified version proposed by Dudarev et al[1]}. The Hubbard U was estimated self-consistently using density functional perturbation theory[2]}. The distance with the nearest periodic image is  1.4 nm, which is enough to discard any kind of interaction between the Fe ions. Generally, Nb atoms carry only small magnetic moments, that are oppositely oriented to the Fe moments. On the lateral edges of the supercell, however, we observe a polarization of Nb atoms which is small and might be influenced by the neighboring Fe sites. All calculations were carried out in the QuantumEspresso code[3]}.
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